The invention relates to a tank filler neck which has an outer wall and an inner wall and, in its interior, forms a space which is closed in relation to the environment during operation and is opened for refueling, wherein a negative and positive pressure relief valve is provided for equalizing the pressure when the space is closed. The tank filler neck may be closed both by means of a screw cap or bayonet cap or may be of the capless type.
In the first case, it can be formed without any inner flaps because the space is closed by the screw or bayonet cap. In the second case, it is equipped at its upper end with a flap which can be pushed open inward by the filling pipe of a fuel nozzle. A further flap—a “lead free flap” can be arranged further inward. The closed space formed in the tank filler neck merges into the interior of the tank and therefore the same pressure as in the tank prevails in it. This pressure fluctuates. When the tank is heated, a positive pressure is produced therein and therefore in the space. When the tank content is cooled and because of the removal of fuel, a negative pressure, which is also undesirable, forms in the tank.
It is therefore known to install a negative and positive pressure relief valve in the cap (for example screw cap) closing off the tank filler neck to the outside in conventional tank systems. Tank caps of this type are described in AT patent 403 144 and DE 102 16 811 A1. They make the tank cap (which is easily lost) expensive and also bulky. In the case of capless tank filler necks, valves of this type are not feasible because the passage of the filling pipe of a fuel nozzle has to be ensured. An inwardly opening flap may be provided in the case of capless tank filler necks. However, when there is positive pressure in the tank, said flap is pressed even more firmly against its seat and, because of the considerable force of its closing spring, cannot equalize a negative pressure in the tank. The fitting of a valve into the flap is out of the question because the flap is pushed open by the filling pipe of the fuel nozzle.
It is therefore the object of the invention to enable an equalization of the pressure in both directions by means of a solution which is as simple and space-saving as possible. The tank cap is to be as simple and inexpensive as possible or entirely superfluous. Equalization of pressure is therefore to be ensured even in the case of capless tank filler necks.